The present invention relates to a honeycomb catalyst body suitably usable for purifying target components such as carbon monoxide (CO), hydro carbonate (HC), nitrogen oxides (NOx), and sulfur oxides (SOx) contained in exhaust gas discharged from stationary engines for automobiles, construction machines, and industry and combustion apparatuses and to a method for manufacturing the honeycomb catalyst body.
At present, a honeycomb structured catalyst body (honeycomb catalyst body) is used for purifying exhaust gas discharged from various kinds of engines, or the like. As shown in FIG. 6, the honeycomb catalyst body has a structure where a catalyst layer 15 is loaded on a surface of partition walls 4 forming cells 3. In addition, as shown in FIGS. 4 and 5, when exhaust gas is purified by the use of the honeycomb catalyst body 60 (honeycomb structure 11), exhaust gas is allowed to flow into the cells 3 of the honeycomb catalyst body 60 from one end face 2a side to bring the exhaust gas into contact with the catalyst layer (not illustrated) on the surface of the partition walls 4, and then the exhaust gas is discharged outside from the other end face 2b (see JP-A-2003-33664).
In the case of purifying exhaust gas using such a honeycomb catalyst body, it is required to accelerate transmission of target components contained in exhaust gas from exhaust gas toward the catalyst layer on the surface of the partition walls as much as possible to enhance purification efficiency. In order to enhance purification efficiency, it is necessary to decrease a hydraulic diameter of the cells, to increase a surface area of the partition walls, and the like. Specifically, there is employed a method of increasing the cell number (cell density) per unit area, or the like.
Here, it is known that transmissibility of target components from exhaust gas toward the catalyst layer on the surface of the partition walls rises in inverse proportion to the square of a hydraulic diameter of the cells. Therefore, as the cell density is increased, the transmissibility of target components rises more. However, pressure loss also tends to increase in inverse proportion to the square of a hydraulic diameter of the cells. Therefore, there arises a problem that pressure loss rises in accordance with rise in transmissibility of target components.
Incidentally, the catalyst layer of the surface of the partition walls generally has a thickness of about several tens μm. Here, when the target components diffuse in the catalyst layer at an insufficient rate, purification efficiency of the honeycomb catalyst body tends to be lowered. This tendency is particularly notable under low-temperature conditions. Therefore, in order to enhance exhaust gas purification efficiency, it is necessary to not only increase the surface area of the catalyst layer, but also reduce thickness of the catalyst layer to raise a diffusion rate of the target components in the catalyst layer. Accordingly, when the cell density is increased, there arises a problem of increasing pressure loss though it has an advantage of increasing the surface area of the catalyst layer.
In order to reduce pressure loss together with enhancing exhaust gas purification efficiency, it is necessary to raise a flow rate of exhaust gas circulating in the honeycomb catalyst body together with increasing an inlet diameter of the honeycomb catalyst body. However, in the case that the honeycomb catalyst body is enlarged, it sometimes makes mounting difficult because of the limited mounting space regarding, for example, a honeycomb catalyst body to be mounted on a vehicle.
The present invention has been made in view of such problems of prior art and aims to provide a honeycomb catalyst body excellent in purification efficiency, having low pressure loss, and mountable even in a limited space and a method for manufacturing the honeycomb catalyst body.
In order to achieve the above aims, the present inventors zealously studied and, as a result, found out that the above aims can be achieved by loading a catalyst layer containing noble metals on an inner surface of the cells of the honeycomb structure and an inner surface of pores in the porous partition walls with specifying a range of mass ratio of the noble metals contained in each of the catalyst layers, which led to the completion of the present invention.